German Pat. No. 16 85 037 describes an arrangement in which between two pairs of surface press rolls a folding unit is provided. The edge portion to be folded is pressed by the rolls and simultaneously transported to the associated folding unit. In this earlier system two folding units are described as having elongated and worm-shaped guides which fold the edge portion during its advance in steps, i.e. the edge portion is firstly bent or raised to an upright position and then folded.
This earlier system has, however, the drawback that the edge portion will be warped with respect to the remaining portion of the fabric when light fabrics are used due to the friction resistance within the folding areas. This is especially true when a double folding is required.
An attempt has been made in German Pat. No. 16 85 041 to avoid this drawback by providing a guiding tongue with an L-shaped rail which guides the edge portion in conveying direction. The guiding tongue cooperates with a disk having a flange portion projecting over the edge of the fabric and which is synchronously driven with the advance movement of the fabric. Consequently the edge portion bent into an upright position will be folded by the flange portion in cooperation with the L-shaped rail.
Although this principle may seem to be sound, it has been shown that upon using primarily light fabrics, the already folded edge portion will be loosened up after leaving the folding units so that such an edge portion will be pressed and reinforced by the subsequent pair of rolls in an incorrect manner.